Silver-refractory metal electrical contact having refractory metal carbide in the marginal layer of its active contact face



United States Patent 3,225,169 SILVER-REFRACTORY METAL ELECTRICAL CON- TACT HAVING REFRACTORY METAL CARBIDE IN THE MARGINAL LAYER OF ITS ACTIVE CONTACT FACE John C. Kosco, St. Marys, Pa., assignor to Stackpole Carbon Company, St. Marys, Pa., a corporation of Pennsylvania No Drawing. Filed Feb. 24, 1964, Ser. No. 347,006 2 Claims. (Cl. 200166) This invention relates to electrical contacts of the type used in, for example, circuit breakers, switches and related make-and-break electrical devices.

' Electrical contacts must meet varied requirements depending upon service conditions. For example, a commonly required property is that they exhibit high electrical conductivity. Another is that they must exhibit resistance to welding during overload and short circuit conditions. In general, such contacts are made presently from silver to supply the requisite conductivity together with a refractory metal or a refractory metal carbide. The choice of the refractory material is determined by the particular device with which it is to be associated, and the various refractory materials contemplated for these purposes have relative to one another various advantages and disadvantages. For instance, the electrical conductivity of tungsten is higher than that of its carbide so that from this standpoint its usewould be advantageous. On the other hand, tungsten carbide is less prone. than tungsten to oxidation during arcing which means that it will maintain a more uniform resistance at the contact face, which is in itself an advantage. In consequence of such factors there are available at the present time no contacts which embody both high conductivity and satisfactory resistance to welding or to undesirable temperature rise as a result of cyclic operation of, for instance, a heavy duty circuit breaker.

It is among the objects of this invention to provide electrical contacts which provide satisfactorily high conductivity coupled with desirable resistance to arcing and welding, and which likewise give superior results in avoiding undesirable temperature rise during cyclic operation, which are of simple composition, and which may be made from readily available materials and by the use of available powder metallurgy practices.

A further object is to provide a method of making such contacts which is simple, easily practiced with standard powder metallurgy equipment, does not require highly skilled operators, and does not involve objectionable modification of existing powder metallurgy equipment or methods.

The present invention is predicated upon my discovery that its stated objects are supplied by an integrated composite contact having a body of silver which provides a matrix for a refractory metal distributed through it, and the contact face of which contains a refractory metal carbide. In this way I am enabled to take advantage of the desirable electrical conductivity of a refractory metal, such as tungsten, and also the advantageous property of a refractory metal carbide, such as tungsten carbide, in minimizing welding at the contact face during overload and short circuit conditions while avoiding the disadvantage of such a carbide when used as the primary or sole refractory material in a silver-base contact.

The invention will be described with particular reference to its application to the production of silver-tungsten contacts. In accordance with it finely divided tungsten is mixed with a temporary binder such as is commonly used in powder metallurgy, examples being paraffin wax, phenolic resins, alkyd resins and other carbonaceous materials which will be volatilized or burned out during sub- 3,225,169 Patented Dec. 21, 1965 sequent processing. The mixture of powder and binder is then placed in a mold and subjected to high pressure to form a compact which is then heated to a sintering temperature in a non-oxidizing atmosphere while supplying carbon to the side of the compact which is to form its active contact face, i.e., the face which is to cooperate with a companion contact member. In this way the active contact face is carburized, with conversion of the tungsten into tungsten carbide at the contact face to a depth of, e.g., 0.001 to 0.010 inch. Production of the contact is then completed by infiltrating it with silver in a non-oxidizing atmosphere, as by placing silver upon the active contact face and heating to a temperature to cause the silver to melt and infiltrate and supply a matrix in which the metallic tungsten of the body and the tungsten carbide in the marginal layer at the active contact face are embedded.

In the practice of the invention the silver and tungsten powders may range from, for example, 20 microns to 1 micron particle size, although the particle size is not critical. The intimate mixture of tungsten and silver powders may be pilled to form compacts at from about 2 to 40 tons per square inch pressure, althoughhere again, the exact pressure is not critical and will depend upon the density desired, the'size of the compact and related factors well known in the powder metallurgy field. Temperatures ranging from 1500 to 1700 C. may be used for sintering the compacts although 1600 C. I believe now to give the best results. Sintering in a non-oxidizing atmosphere can be accomplished in various ways, as by sintering in a vacuum, or in a reducing or inert gas atmosphere, as willbe understood by those concerned with the art of powdermetallurgy.

Carburizing the contact face of the compact is accomplished most simply by supporting that face of the compact on a body of carbon, such as graphite, during the sintering operation;. other forms of carbon may be used likewise. Alternatively, carburization can be achieved by supplying a carburizing atmosphere, of which a variety are known, to the contact face during sintering.

Infiltration with silver as alluded to above is accomplished by heating the compact with silver in contact with one face at a temperature of l250 to 1600 C., the exact temperature not being critical.

When a contact made asjust described is examined metallographically it is found that the body of the contact consists of particles of tungsten uniformly embedded in a matrix of silver, while the surface that is to be the active contact face exhibits particles of tungsten carbide distributed through a silver matrix, with the whole constituting an integral composite.

For the purposes of the invention such tungsten-containing contacts may contain from about 30 to about 65 volume percent of silver with the remainder consisting essentially of metallic tungsten a very small proportion of which at the contact face is in the form of. tungsten carbide. 7

Although the invention has been described with particular reference to silver-tungsten contacts, silver-base contacts may be formed similarly from other carbideforming metals such as molybdenum, titanium and silicon. In such instances factors such as particle size, type of binder, pilling pressure, atmosphere, sintering and infiltration temperatures Will in general follow those described above, any variation therefrom being readily determinable in any given instance. In the case of these refractory metals also the silver may range from about 30 to about 65 percent by volume.

Contacts made from silver and tungsten as described above readily pass Underwriters Laboratories electrical tests including endurance, overload, short circuit, and dielectric strength tests.

Recently some companies have included a further test requirement on contact materials, particularly those which will be used in switching devices. This involves cycling the breaker at rated current and measuring the temperature rise at the stationary terminal, and to pass this test a maximum rise of 50 C. above ambient is permitted. According to reports, commercial silver-tungsten materials will not meet this temperature rise specification unless the contacts are given a silver plating about 0.001 to 0.002 inch thick, and even with this treatment temperature rises are of the order of 45 C.

As exemplfying the utility of contacts made in accordance with this invention, in addition to passing the standard Underwriters Laboratories tests, the contacts of this invention exhibit remarkably low temperature rise at the stationary terminal in the cycling test, as evidenced by the following results which were obtained with a Westinghouse QuicklagC, 40 C, Cat. No. QCL1020 breaker in which contacts in accordance with this invention were installed. Temperature readings were obtained with a thermocouple bolted to the stationary contact wiring terminal. The breaker was mounted in air and connected to the power supply with not less than 4 feet of insulated conductor per terminal, the wire size corresponding to the breaker size. The test was run with the breaker operated six times per minute at 120 volts, 60 cycles, rated current and unity power factor. The results of two such tests are tabulated below and show that the temperature rise above ambient was far below the maximum required by the companies using this test:

Temperature Wiring Ambient Terminal in Deg.

Temp. Centigrade in Deg. Centigrade Test N0. Test No. 333 334 Operating test, start. 27 27 27 After 430 cycles.. 29. 35 34 After 790 cycles.. 29. 5 35. 5 37. 5 After 1,150 cycles 29 36 36 After 1,860 cycles.. 29 35 34 After 2,140 cycles. 29 35 34 After 2,400 cycles. 29 35 34 After 2,800 cycles. 28 34 34 After 8,573 cycles.. 24. 5 33 33. 5 After 9,238 cycles. 26. 5 35. 5 36 After 9,678 cycles.. 26. 5 35. 5 36 After 10,360 cycles. 27. 0 36 36. 5

Moreover, in a continuous current test under the same conditions the temperature rise was less than 20, equally demonstrating the value of the contacts of this invention:

According to the provisions of the patent statutes, have explained the principle of my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. An electrical contact having an active contact face, the contact consisting essentially of a sintered body of silver, a finely divided metal of the group consisting of tungsten, molybdenum, titanium and silicon, and a carbide of said metal, the silver providing a matrix through which said metal is distributed, and said carbide being present in and restricted to the exposed marginal layer of said contact face, the silver consisting of from about 30 to volume percent of the contact with the remainder of the contact said metal and said carbide.

2. An electrical contact according to claim 1, said metal being tungsten and said carbide being tungsten carbide.

References Cited by the Examiner UNITED STATES PATENTS 2,289,708 7/1942 Jackson 200-166 2,298,999 10/1942 Allen 200166 2,300,286 10/1942 Gwyn 200-166 2,624,820 1/1953 Payette 200166 3,026,603 3/1962 Zysk et al. 29l55.55 3,139,669 7/1964 GWyn 29155.55

KATHLEEN H. CLAFFY, Primary Examiner.

BERNARD A. GILHEANY, Examiner. 

1. AN ELECTRICAL CONTACT HAVING AN ACTIVE CONTACT FACE, THE CONTACT CONSISTING ESSENTIALLY OF A SINTERED BODY OF SILVER, A FINELY DIVIDED METAL OF THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, TITANIUM AND SILICON, AND A CARBIDE OF SAID METAL, THE SILVER PROVIDING A MATRIX THROUGH WHICH SAID METAL IS DISTRIBUTED, AND SAID CARBIDE BEING PRESENT IN AND RESTRICTED TO THE EXPOSED MARGINAL LAYER OF SAID CONTACT FACE, THE SILVER CONSISTING OF FROM ABOUT 30 TO 65 VOLUME PERCENT OF THE CONTACT WITH THE REMAINDER OF THE CONTACT SAID METAL AND SAID CARBIDE. 